arXiv subjects

L. J. Abu-Raddad

Publications and source records attributed to L. J. Abu-Raddad.

11 recordsLinked to original sources

arXiv2005

The mass of the nucleon in a chiral quark-diquark model

The mass of the nucleon is studied in a chiral quark-diquark model. Both scalar and axial-vector diquarks are taken into account for the construction of the nucleon state. After the hadronization procedure to obtain an effective meson-baryon Lagrangian, the quark-diquark self-energy is calculated in order to generate the baryon kinetic term as well as the mass of the nucleon. It turns out that both the scalar and axial-vector parts of the self-energy are attractive for the mass of the nucleon. We investigate the range of parameters that can reproduce the mass of the nucleon.

hep-ph↗

arXiv2003

Quasifree eta photoproduction from nuclei and medium modifications of resonances

We investigate the sensitivity of the differential cross section, recoil nucleon polarization and the photon asymmetry to changes in the elementary amplitude, medium modifications of the resonance $(S_{11},D_{13})$ masses, as well as nuclear target effects. All calculations are performed within a relativistic plane wave impulse approximation formalism resulting in analytical expressions for all observables. The spin observables are shown to be unique tools to study subtle effects that are not accessible by only looking at the unpolarized differential cross section.

nucl-th↗

arXiv2002

The nucleon and the nuclear force in the context of effective theory and path-integral methods

The nucleon structure and the nuclear force are investigated in the context of the non-perturbative path-integral method of hadronization. Starting from a microscopic quark-diquark model, the nucleon is generated as a relativistic bound state and an effective chiral meson-nucleon Lagrangian is derived. Many of the nucleon physical properties are studied using a theory of at most two free parameters.

nucl-th↗

arXiv2002

Pion-nucleus optical potential valid up to the DELTA-resonance region

We present in this article an optical potential for the $π$-nucleus interaction that can be used in various studies involving $π$-nucleus channels. Based on earlier treatments of the low energy $π$-nucleus optical potential, we have derived a potential expression applicable from threshold up to the $Δ$-resonance region. We extracted the impulse approximation form for this potential from the $π-N$ scattering amplitude and then added to it kinematical and physical corrections. The kinematic corrections arise from transforming the impulse approximation expression from the $π-N$ center of mass frame to the $π$-nucleus center of mass frame, while the physical corrections arise mostly from the many-body nature of the $π$-nucleus interaction. By taking advantage of the experimental progress in our knowledge of the $π-N$ process, we have updated earlier treatments with parameters calculated from state-of-the-art experimental measurements.

nucl-th↗

arXiv2002

Path-integral hadronization for the nucleon and its interactions

Nucleon structure and the origin and nature of the nuclear force are investigated in the context of a QCD-based effective field theory and the path-integral method of hadronization. We start from a microscopic model of quarks and diquarks where the gluons have been integrated out. In particular, we use the chiral Nambu-Jona-Lasinio model to describe quark dynamics and assume that the nucleon can be conceived as a quark-diquark relativistic bound state. The hadronization method is then used to rewrite the problem in terms of the physical meson and nucleon degrees of freedom. Next, by employing a loop and derivative expansion of the resulting quark/diquark determinants, we arrive at an effective chiral meson-nucleon Lagrangian. Nucleon properties such as mass, coupling constants, electromagnetic radii, anomalous magnetic moments, and form factors are derived using a theory of at most two free parameters.

nucl-th↗

arXiv2002

Quasifree processes from nuclei: Meson photoproduction and electron scattering

We have developed a relativistic formalism for studying quasi-free processes from nuclei. The formalism can be applied with ease to a variety of processes and renders transparent analytical expressions for all observables. We have applied it to kaon photoproduction and to electron scattering. For the case of the kaon, we compute the recoil polarization of the lambda-hyperon and the photon asymmetry. Our results indicate that polarization observables are insensitive to relativistic, nuclear target, and distortion effects. Yet, they are sensitive to the reactive content, making them ideal tools for the study of modifications to the elementary amplitude -- such as in the production, propagation, and decay of nucleon resonances -- in the nuclear medium. For the case of the electron, we have calculated the spectral function of He-4. An observable is identified for the clean and model-independent extraction of the spectral function. Our calculations provide baseline predictions for the recently measured, but not yet fully analyzed, momentum distribution of He-4 by the A1-collaboration from Mainz. Our approach predicts momentum distributions for He-4 that rival some of the best non-relativistic calculations to date.

nucl-th↗

arXiv2002

Introduction to Effective Field Theories in QCD

We present a simple introduction to the techniques of effective field theory (EFT) and their application to QCD. For problems with more than one energy scale, the EFT approach is a useful alternative to more traditional model-building strategies. The most relevant such problem for this discussion is that of making contact between QCD and the hadronic phase of matter. As a simple example, an EFT calculation of the bound states of hydrogen within QED is sketched. A more significant demonstration of the power of EFTs, the construction of the chiral Lagrangian and chiral perturbation theory, is also included. The results provide us with the road map to a complete QCD-based theory of nuclear matter at nonzero temperatures and densities, a vital component to a quantitative understanding of the phase transition from hadron gas to quark-gluon plasma.

nucl-th↗

arXiv2001

Extracting the spectral function of He-4 from a relativistic plane-wave treatment

The spectral function of He-4 is extracted from a plane-wave approximation to the (e,e'p) reaction using a fully relativistic formalism. We take advantage of both an algebraic ``trick'' and a general relativistic formalism for quasifree processes developed earlier to arrive at transparent, analytical expressions for all quasifree (e,e'p) observables. An observable is identified for the clean and model-independent extraction of the spectral function. Our simple relativistic plane-wave calculations provide baseline predictions for the recently measured, but not yet fully analyzed, momentum distribution of He-4 by the A1-collaboration from Mainz. Yet in spite of its simplicity, our approach predicts momentum distributions for He-4 that rival some of the best nonrelativistic calculations to date. Finally, we highlight some of the challenges and opportunities that remain, both theoretically and experimentally, in the extraction of quasifree observables.

nucl-th↗

arXiv1999

Quasifree kaon-photoproduction from nuclei in a relativistic approach

We compute the recoil polarization of the lambda-hyperon and the photon asymmetry for the quasifree photoproduction of kaons in a relativistic impulse-approximation approach. Our motivation for studying polarization observables is threefold. First, polarization observables are more effective discriminators of subtle dynamics than the unpolarized cross section. Second, earlier nonrelativistic calculations suggest an almost complete insensitivity of polarization observables to distortions effects. Finally, this insensitivity entails an enormous simplification in the theoretical treatment. Indeed, by introducing the notion of a ``bound-nucleon propagator'' we exploit Feynman's trace techniques to develop closed-form, analytic expressions for all photoproduction observables. Moreover, our results indicate that polarization observables are also insensitive to relativistic effects and to the nuclear target. Yet, they are sensitive to the model parameters, making them ideal tools for the study of modifications to the elementary amplitude --- such as in the production, propagation, and decay of nucleon resonances --- in the nuclear medium.

nucl-th↗

arXiv1998

Lessons to be learned from the coherent photoproduction of pseudoscalar mesons

We study the coherent photoproduction of pseudoscalar mesons---particularly of neutral pions---placing special emphasis on the various sources that put into question earlier nonrelativistic-impulse-approximation calculations. These include: final-state interactions, relativistic effects, off-shell ambiguities, and violations to the impulse approximation. We establish that, while distortions play an essential role in the modification of the coherent cross section, the uncertainty in our results due to the various choices of optical-potential models is relatively small (of at most 30%). By far the largest uncertainty emerges from the ambiguity in extending the many on-shell-equivalent representations of the elementary amplitude off the mass shell. Indeed, relativistic impulse-approximation calculations that include the same pionic distortions, the same nuclear-structure model, and two sets of elementary amplitudes that are identical on-shell, lead to variations in the magnitude of the coherent cross section by up to factors of five. Finally, we address qualitatively the assumption of locality implicit in most impulse-approximation treatments, and suggest that the coherent reaction probes---in addition to the nuclear density---the polarization structure of the nucleus.

nucl-th↗

arXiv1997

Nuclear dependence of the coherent eta photoproduction reaction in a relativistic approach

We study the nuclear (or A) dependence of the coherent eta photoproduction reaction in a relativistic impulse approximation approach. We use a standard relativistic parameterization of the elementary amplitude, based on a set of four Lorentz- and gauge-invariant amplitudes, to calculate the coherent production cross section from He-4, C-12, and Ca-40. In contrast to nonrelativistic treatments, our approach maintains the full relativistic structure of the process. The nuclear structure affects the process through the ground-state tensor density. This density is sensitive to relativistic effects and depends on A in a different manner than the vector density used in nonrelativistic approaches. This peculiar dependence results in He-4 having a cross section significantly smaller than that of C-12---in contrast to existent nonrelativistic calculations. Distortion effects are incorporated through an eta-nucleus optical potential that is computed in a simple ``t rho'' approximation.

nucl-th↗